I have 3 carver pm1.5 amps that are the same age, one of them has recently started having a problem passing signal when first powered up. it's just one channel of one amp. If the input level is low, there is no output. If I raise the level of the incoming signal, it reaches a point where it sounds like an input capacitor starts passing signal, and the audio seems fine. If the amp is cold, then the amp will stop passing signal when then input level drops below a point. If the amp is warm, it happens much less. I would guess that an input cap, somewhere in the path, is dried out and not passing signal very well. My question is, does anybody have a schematic (or experience) and can tell me which caps to start with?

Hi Chris,
These amps are totally different to what most service people are used to looking at. Then there are DIY type people, like yourself I assume.

So, some warnings are in order:

1. These amps don't discharge the power supplies when off.
2. The high supply rails are anywhere from 118 to 125 VDC or higher!!
3. It's easy to complete a circuit with your knuckle when working on other things.
4. The triac is not used as an on - off type control. It's an input power regulator.
5. The power transformer is not a normal transformer.
6. These amps are pretty old by now. Multiple problems are expected, even if you haven't noticed them yet.

I highly recommend that an ex-warranty (for Carver!) technician work on this only. No TV guys, no car amp guys, no weekend warriors. I'm not trying to be elitist here. There is very real possibilities of you causing damage to these amps or to yourself. In particular, the voltage adjustment control can easily be set to damage the amplifier. Your voltage is already probably too high.

As far as the actual fault that you are experiencing right now is concerned, it's likely one or more bad solder connections. There is a coupling capacitor about 1/4 the way from the "mother board" near the top on each amplifier card. Most are green, but you can see it's a film capacitor laid over and fixed with glue. Often the leads have defective solder. Remove this component and clean the leads, install and solder again. You may also want to replace this part with one of similar value. Use a film type capacitor here. The other areas are the connections from the amplifier cards to the "mother board". This is where you can really get into trouble and, unless you are very experienced and careful, please don't attempt to work in these areas.

There are many other potential problems with these amplifiers that may cause larger failures unless they are looked after properly. These can be tricky to solve, just the mechanics of working where the parts are located. You won't know there is a problem until a more dramatic failure occurs.

I use a 1/8" wide screwdriver tip at about 700°F~800°F, I've wicked the part and pad dry, and then re-soldered.

Still doesn't work.

The solder joints looked perfect before I started, and looked perfect when I finished.

It was only when I scaped through to the steel lead core, and re-soldered with an acid flux that the problems went away.

I have had the same problems with 5% carbon films, again steel leads (copperweld). You have to scape through to the steel core to get reliable operation.

I was seeing this as seemingly random (at first), and then I began to see a pattern.

I need to finish up a replacement driver card design for Hafler driver boards, I've wasted too much time and energy trying to fix intermittents on these. In the long run it will be more cost effective to replace with a new board.

__________________
Candidates for the Darwin Award should not read this author.

I have had the same problems with 5% carbon films, again steel leads (copperweld). You have to scape through to the steel core to get reliable operation.

It's part of my normal way of servicing to remove parts with bad solders and clean the leads before re-soldering. My thought was oxidation on the exposed surfaces. If my thoughts were incorrect on the causes, the actions would correct the problem either way. I always use extra solder flux as well. I don't care whether the parts in question are resistors, diodes, transistors or capacitors. The leads always require cleaning to be confident of a good connection.

Quote:

I need to finish up a replacement driver card design for Hafler driver boards, I've wasted too much time and energy trying to fix intermittents on these. In the long run it will be more cost effective to replace with a new board.

You are probably on to something. Time is the single most costly portion of a repair. If PCBs have a history of being problematic, replacing them is a good option. You can then perform some fine tuning and upgraded components. Instant upgrade with parts that actually fit.